1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor light-emitting device having a light emission output of high light intensity.
The present application is based on Japanese Patent Application No. Hei. 11-090718, which is incorporated herein by reference.
2. Description of the Related Art
A device of a double heterostructure for emitting green or blue light is known as a light-emitting device comprising layers of group III nitride compound semiconductors laminated on a substrate. For example, a light-emitting device with a p-type clad layer made of AlxGa1xe2x88x92xN (0 less than x less than 1) is generally known. In this type light-emitting devices, a p-type contact layer was generally heretofore made of gallium nitride (GaN).
When a p-type contact layer made of gallium nitride (GaN) was grown on a p-type clad layer made of AlxGa1xe2x88x92xN (0 less than x less than 1), the difference both in layer thermal expansion coefficient and in crystal lattice constant between the p-type clad layer and the p-type contact layer increased as the value of the composition ratio x increased.
If the difference in crystal lattice constant between the two layers became large, the p-type contact layer was hardly grown as a layer of good quality. This caused reduction in intensity of emitted light.
On the other hand, if the difference in thermal expansion coefficient between the two layers became large, distortion due to the difference in thermal expansion coefficient was caused in an epitaxial wafer when the temperature was decreased from a high temperature to a room temperature after crystal growth. As a result, stress remained in the epitaxial wafer, so that this caused reduction in intensity of emitted light.
The present invention is designed to solve the aforementioned problem and an object thereof is to provide a light-emitting device of high light intensity by eliminating the disadvantage caused by the difference both in thermal expansion coefficient and in lattice constant between the aforementioned two layers.
The following means are effective for solving the problem.
That is, as a first means, there is provided a semiconductor light-emitting device comprising layers of group III nitride compound semiconductors laminated on a substrate, and a p-type clad layer of AlxGa1xe2x88x92xN (0 less than x less than 1), is in that the device further comprises a p-type contact layer made of AlyGa1xe2x88x92yN (0 less than y less than x) which is lower in the composition ratio of aluminum (Al) than the p-type clad layer.
As a second means, preferably, in the first means, the p-type contact layer is made of AlyGa1xe2x88x92yN (0.1xxe2x89xa6yxe2x89xa60.7x). More preferably, the value of the composition ratio y of aluminum (Al) in the p-type contact layer is approximately in the range xe2x80x9c0.4xxe2x89xa6yxe2x89xa60.5xxe2x80x9d.
As a third means, preferably, in first means, the p-type contact layer is made of AlyGa1xe2x88x92yN (0.01xe2x89xa6yxe2x89xa60.12). More preferably, the absolute value of the composition ratio y of aluminum (Al) in the p-type contact layer is approximately in the range xe2x80x9c0.03xe2x89xa6yxe2x89xa60.08xe2x80x9d.
As a fourth means, preferably, in any one of the first, second and third means, the thickness of the p-type contact layer is selected to be in a range of from 200 xc3x85 to 1000 xc3x85 both inclusively. More preferably, the thickness of the p-type contact layer is selected to be in a range of from 500 xc3x85 to 800 xc3x85 both inclusively.
The aforementioned problem can be solved by the above means.
Incidentally, the group III nitride compound semiconductors according to the present invention are represented by the general formula AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61), which may further contain group III elements such as boron (B) and thallium (Tl) and in which the nitrogen (N) may be partially replaced by phosphorus (P), arsenic (As), antimony (Sb) or bismuth (Bi).
Accordingly, each of layers such as a buffer layer, a barrier layer, a well layer, a clad layer, a contact layer, an intermediate layer, a cap layer, etc. in the group III nitride compound semiconductor light-emitting device may be made of quaternary, ternary or binary AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61), such as AlGaN, InGaN, or the like, of an optional crystal mixture ratio.
Further, a metal nitride compound such as titanium nitride (TiN), hafnium nitride (HfN), or the like, or a metal oxide compound such as zinc oxide (ZnO), magnesium oxide (MgO), manganese oxide (MnO), or the like, other than the aforementioned group III nitride compound semiconductor may be used as the buffer layer.
Further, a group II element such as beryllium (Be), zinc (Zn), or the like, other than magnesium (Mg) may be used as the p-type impurities.
Further, the n-type semiconductor layer may be formed by doping the aforementioned group III nitride compound semiconductor with a group IV element such as silicon (Si), germanium (Ge), or the like, or with a group VI element.
Further, silicon carbide (SiC), zinc oxide (ZnO), magnesium oxide (Mgo), manganese oxide (MnO), or the like, other than sapphire may be used as the substrate for crystal growth.
According to the means of the present invention, the composition of the p-type contact layer becomes close to that of the p-type clad layer. Hence, the difference in crystal lattice constant between the two layers is reduced, so that the p-type contact layer of good quality can be grown. Hence, the intensity of light emitted from the light-emitting device is improved.
According to the means of the present invention, the difference in thermal expansion coefficient between the two layers is reduced as well. Hence, stress remaining in the epitaxial wafer is reduced, so that the intensity of emitted light is improved.
When a p-type contact layer of AlyGa1xe2x88x92yN (0 less than y less than x), which is lower in the composition ratio of aluminum (Al) than a p-type clad layer of AlxGa1xe2x88x92xN (0 less than x less than 1), is grown on the p-type clad layer, the value of the composition ratio y of aluminum (Al) is selected to be preferably approximately in the range xe2x80x9c0.1xxe2x89xa6yxe2x89xa60.7xxe2x80x9d, more preferably in the range xe2x80x9c0.4xxe2x89xa6yxe2x89xa60.5xxe2x80x9d. If the value of the composition ratio y in the p-type contact layer is too large, contact resistance between the positive electrode and the p-type contact layer increases, undesirably resulting in an increase in the drive voltage of the light-emitting device. If the value of the composition ratio y is too small, it is difficult to obtain the aforementioned operation and effect of the present invention because the composition of the p-type contact layer is not close to that of the p-type clad layer.
For the same reason as described above, the absolute value of the composition ratio y of aluminum (Al) in the p-type contact layer is also selected to be preferably approximately in the range xe2x80x9c0.01xe2x89xa6yxe2x89xa60.12xe2x80x9d, more preferably in the range xe2x80x9c0.03xe2x89xa6yxe2x89xa60.08xe2x80x9d. More in detail, the group III nitride compound semiconductor light-emitting device exhibits the highest intensity of emitted light particularly when the absolute value of the composition ratio y is about 0.05.
Though will be described later in detail, there is a strong correlation between the light emission output of the light-emitting device and the thickness of the p-type contact layer as shown in FIG. 2. Hence, the thickness of the p-type contact layer is preferably in a range of from 200 xc3x85 to 1000 xc3x85 both inclusively. More preferably, the thickness of the p-type contact layer is in a range of from 500 xc3x85 to 800 xc3x85 both inclusively. When the thickness of the p-type contact layer is in this range, the light emission output of the light-emitting device exhibits a large value. Further, the group III nitride compound semiconductor light-emitting device according to the present invention exhibits the highest intensity of emitted light particularly when the thickness of the p-type contact layer is about 600 xc3x85.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.